At the present time, it is desirable to provide filtration apparatus for removing virus from biological fluids which are presterilized at the site of manufacturing the filtration apparatus and sanitized at the site of using the filtration apparatus. Generally, sterilization at the manufacturing site is effected with gamma radiation and sanitization is effected with caustic at the site of use. Caustic sanitization is effected first with a caustic aqueous solution (typically 0.1 normal sodium hydroxide), then with sterile water followed by sterile buffer. Accordingly, it is necessary that the filtration apparatus be resistant to degradation due to both gamma radiation and caustic.
The filtration apparatus utilizes elements comprising a support plate to which is bonded a filtration membrane generally comprising a polysulfone (PS) membrane or a polyethersulfone (PES) membrane since high performance ultrafiltration (UF) can be effected therewith.
At the present time, it is known that PS or PES membranes can be heat bonded to an acrylic substrate. Since acrylic is not caustic resistant, its use in processes that requires caustic sterilization is undesirable.
PS or PES membrane can be easily thermally bonded to a polysulfone substrate (plate) because of the similarity of the materials. However, it is difficult to bond the plates together. A process typically used to bond plates together is called contact welding. The process involves putting two plastic plates in contact with a heater until the plastic begins to melt. The plates are then removed from the heater, the heater is slid out of the way and the plates are pressed together. This process requires the plastic material to remain molten until the two plates are pressed together so that, when the material cools, the two plates are completely welded. It is difficult to weld a polysulfone or polyethersulfone plate to a plate of the same or similar material using this process. It has been found that melted PS or PES quickly forms a skin when the heating element is removed therefrom which renders formation of a good plate to plate bond difficult.
Other methods of welding PS or PES plates together have been attempted and proven to be difficult or produce some negative effect on the finished product. Some of these methods include laser welding, radio frequency (RF) welding, vibration welding, ultrasonic welding and solvent welding. Laser and RF welding do not produce a sufficiently strong bond. Vibration and ultrasonic welding result in a generation of PS or PES particles that is undesirable. Solvent welding with a solvent like methylene chloride is difficult because the membrane is made of a similar material and can be changed or dissolved by the solvent.
Accordingly, it would be desirable to provide elements of a filtration cartridge which include a membrane heat bonded to a substrate that can be contact welded and have none of the negative effects discussed above. In addition, it would be desirable to provide such elements which are also resistant to degradation by exposure to either gamma radiation or caustic.